Effects of the Initial Gel Fuel Temperature on the Ignition Mechanism and Characteristics of Oil-Filled Cryogel Droplets in the High-Temperature Oxidizer Medium

Abstract

The ignition mechanism was studied for a group of gel fuel compositions in a high-temperature oxidizer medium. It was determined how the initial temperature of the fuel influences the ignition characteristics. The gel fuel (oil-filled cryogel) was prepared from an oil emulsion based on the mixture of a combustible liquid and polyvinyl alcohol. The composition of primary oil emulsions was as follows: the aqueous solution of polyvinyl alcohol (5, 10 wt %) + 40–60 vol % of oil + 2 vol % of emulsifier. The initial temperature of gel fuels ranged from 188 to 293 K. Combustion was initiated in high-temperature motionless air at 873–1273 K. Using a high-speed video recording system, we established that at different initial temperatures of the gel fuel, a set of identical processes occurs during the induction period; these are different from the same physical and chemical processes during the ignition of a combustible liquid. After reaching threshold conditions, the flame spreads in the droplet’s vicinity from a hot spot through the gas mixture. Hot spot is an ignited and a small-sized fragment separating and moving away from the molten fuel droplet as a result of a microexplosion. The values of the main process characteristicignition delay timesdiffer 25–95% for fuel samples with the initial temperature of 293 K and temperatures of 188–233 K because of a long heating and melting stage of the latter. This is explained by a 2.5–3.6-fold difference in the amount of energy, which is necessary to supply to a colder fuel sample for this phase transformation to occur, other things being equal.

Critical Conditions for the Ignition of a Gel Fuel under Different Heating Schemes

“…As a result of the numerical modeling of the process being researched, the ignition delay times of the gel fuel were obtained. These values are in satisfactory agreement with the corresponding experimental data in [33] (Figure 4) where the temperature T varies between 973 and 1273 K, and the initial fuel temperature equals 203 K. The difference between the theoretical and experimental values of ignition delay times does not exceed the measurement error τ in the experiments in [33]. Based on the satisfactory agreement between the results of the mathematical simulation and the experimental data in [33], it can be concluded that the developed mathematical model adequately describes a group of interdependent physicochemical processes taking place during the ignition of gel fuel in a high-temperature environment.…”

“…For time discretization, a step of 10 −3 s was used. This step corresponded to the accuracy of the experiment [33], the data of which were used in this work.…”

Effect of Shear History on Solid–Liquid Transition of Particulate Gel Fuels

“…Gel fuels filled with composite particles possess the enhanced rheo-physical property and better performance of combustion and explosion [1][2][3], such as higher combustion rates [4,5] and catalytic decomposition [6,7]. These benefits allow gel fuels to be used for a wider range of advanced propulsion systems.…”